The transmission, routing and dissemination of information has occurred over computer networks for many years via standard electronic communication lines. These communication lines are effective, but place limits on the amount of information being transmitted and the speed of the transmission. With the advent of light-wave technology, a large amount of information is capable of being transmitted, routed and disseminated across great distances at a high rate over communication lines made of optical fiber. An optical fiber is a strand of glass about the same diameter as a human hair. Light travels down the core of the fiber reflecting off a mirror-like interface where the glass core and a protective sleeve or cladding meet.
In most fiber systems, dust and other contaminants are not a major problem as long as the “light”, that is the optical signal, remains within the optical fiber. However, problems arise when the optical signal must pass from one fiber to another or where the optical signal must leave the fiber and enter a receiver or piece of test equipment, such as an optical power meter.
The most common mechanical arrangement to allow light to travel from one fiber to another is an optical connector. Fiber optic connector systems are designed to align two fiber ends so that the light signal will pass between them. A variety of connector types have evolved such as FC/PC, FC/APC, ST and SC, each with a distinct area of application and all known in the art.
FC/PC and FC/APC connectors are most commonly found in high-end singlemode fiber telecommunications systems. The term “FC” is a fiber connector designated by NTT. “PC” and “APC” describe the kind of polish applied to the connector end face. PC stands for physical contact. A PC connector has a polished convex end face. SPC and UPC are “super” polished and “ultra” polished with better back reflection specification than PC. APC stands for an angled physical contact. An APC connector has a polished end face angled at 8°.
ST and SC connectors are used in commercial wiring and are frequently used in multimode fiber applications in building and campus LAN cabling systems. ST or “standard termination” connectors use a twist on-twist off type of housing. SC or “standard connection” connectors use a push to snap on, and a push to snap off type of housing.
Most connector systems restrain the two fibers to be coupled within precision ferrules, which in turn are held in place by a housing. Within the housing, a precision alignment sleeve aligns the two ferrules and thus the two fibers. The fiber ends are flush with the ferrule ends and are polished to reduce loss of light. All modern connector designs involve physical contact between the two fiber ends. Loss of light at a connection is called insertion loss or attenuation and is measured in dB. Typically, attenuation for a mated pair of high quality connectors is 0.35 dB or less.
The optical return loss, which describes the amount of light reflected from the connection is less than −45 dB for PC, less than −55 dB for UPC, and less than −65 dB for APC.
Dust, dirt and other contaminants are a problem in such optical connections because they interfere with the passage of light from one fiber to another. Fiber optic connectors must be kept clean to ensure long life and to minimize transmission loss and optical return loss at the connection point. A single dust particle caught between two connectors will cause significant signal loss. Dust particles as small as 1 μm diameter on the optical fiber end can significantly degrade performance. Particles 8 μm in diameter or larger on the end of the core can cause a complete failure of the optical system.
Even worse, dust particles can scratch the polished fiber end, resulting in permanent damage. Because the fiber end areas make physical contact, if a connector is mated while contaminated, especially with hard or abrasive contaminant particles, fiber end damage may occur, or the contaminant may get firmly bonded to the fiber end. This can lead to permanent physical damage to the fiber ends, which will necessitate replacement of the connector.
In order to avoid problems and to keep fiber ends in peak condition, connector cleaning must be undertaken frequently. Inspection of the fiber end quality also needs to be undertaken to determine if cleaning is required or if the connector is seriously damaged.
The two basic approaches to cleaning are wet and dry cleaning. Wet cleaning utilizes a solvent such as Isopropyl Alcohol and fiber optic swabs. The swabs have a head made of a soft nonabrasive material that has low particle and fiber generation. Dry cleaning takes a number of forms, but the most common approach involves a special alcohol-free cloth or textile cleaning tape or film on a reel inside a cartridge. The cartridge stores the tape reel and provides a window onto a short portion of the cloth tape for cleaning the fiber ends of connectors. Also, a fiber optical swab with a sticky or tacky head may be used. Dynamic cleaning devices are also available which “spin” a cleaning cloth across the end of the fiber.
Microscopic inspection must be conducted to confirm that cleaning is successful. On some occasions, even after repeated cleaning, inspection will show that the fiber end is damaged beyond recovery and the connector must be replaced. A variety of fiber connector inspection microscopes are available such as Hi-Tech Electronics Series 124000 borescopes, fiberscopes, and videoscopes. The general term “scope” will be used to refer to borescope, fiberscope and videoscope.
Optical connectors are normally removed and cleaned individually by hand. However, in recent years, it has become common to utilize multiple optical connectors composed of fiber arrays which attach to a backplane examples are the “MPX” from TYCO or the “HBMT” from MOLEX. A backplane is an electronic circuit board containing circuitry and sockets into which additional electronic devices on other circuit boards or cards can be plugged.
Optical connectors on a backplane are difficult to clean for several reasons. One reason is because the equipment housing the backplane and optical connectors is generally placed against a wall making access to the rear of the optical connectors difficult. Also, the optical connectors are small and difficult to handle when removed for cleaning so it is advantageous to be able to leave them in the system and clean the optical connectors via front access to the shelf. Another reason is that cleaning the optical connectors via front creates the possibility of contact with other system components causing catastrophic failure of the system. Another reason is that once the cleaning process is complete, all the fiber ends in the bundle must be clean.
Furthermore, due to the small scale of the optical connector, finding the correct position for the cleaning swab or inspection scope while viewing the backplane from the front is very difficult.
What is needed is a simple and reliable apparatus and method for cleaning and inspecting optical connectors inside a shelf where the connectors are not easily accessible. What is also needed is a simple and inexpensive mechanical system to position a cleaning device or inspection scope close to a connector on the backplane, while shielding other system components.